In a synchronous motor using a permanent magnet, particularly as for a motor in which stator windings are wound intensively around teeth, a ratio between the number of poles of the permanent magnet to be used for a rotor and the number of slots (that is, the number of teeth) of a stator is generally 2:3.
In the synchronous motor in which the ratio between the number of poles and the number of slots of the stator is 2:3, an opening is often provided between the adjacent teeth. The opening is provided to facilitate interlinkage of magnetic fluxes generated by a permanent magnet positioned in a rotor with stator windings, and to prevent that the magnetic fluxes generated because of an electric current flowing to the stator windings from short-circuiting in the stator without flowing to the rotor. However, near the opening, because magnetic flux density distribution in a gap between the stator and the rotor is disturbed, cogging torque is generated, and this cogging torque causes vibration and noise.
To reduce such cogging torque, in a conventional synchronous motor, a rotor having 8 or 10 poles and a stator having 9 slots are used, and three-phase windings are continuously and intensively positioned in the stator. In this type of synchronous motor, because nine windings are positioned in the stator, respective windings and teeth are mechanically positioned for every 40 degrees, and three windings of respective phases are mechanically positioned at an angle of 40 degrees continuously. Because the rotor has 8 poles or 10 poles, in the case of 8 poles, the width of one magnetic pole is 45°, and in the case of 10 poles, the width of one magnetic pole is 36°.
To be specific, pulsations generated while the rotor rotates once are determined by the least common multiple of the number of slots of the stator and the number of poles of the rotor. In a case where the synchronous motor having the ratio between the number of poles and the number of slots of the stator of 2:3 is an 8-pole/12-slot synchronous motor, the pulsations occur 24 times. Meanwhile, in an 8-pole/9-slot synchronous motor, the pulsations occur 72 times, and in a 10-pole/9-slot synchronous motor, the pulsations occur 90 times.
As described above, as the number of pulsations increases, energy of the cogging torque is distributed more, and thus the amplitude of the cogging torque decreases. That is, the 8-pole/9-slot or 10-pole/9-slot synchronous motor can suppress the cogging torque more than the synchronous motor in which the ratio between the number of poles and the number of slots of the stator is 2:3.
However, in the above-discussed conventional synchronous motor, because the winding constituting one phase is positioned continuously and intensively, a rotating field generated by distributing an electric current to the stator winding is generated unequally with respect to a rotary shaft of the rotor. Therefore, an attracting and repelling force between the permanent magnet of the rotor and the stator becomes unbalanced with respect to the rotary shaft, thereby generating a large excitation force in a radial direction. This excitation force causes vibration and noise.